1. Field of the Invention
This invention relates to the fabrication of microwave components and specifically to the fabrication of corrugated or ridged microwave components.
2. Description of the Prior Art
Corrugated or ridged feeds, horns, waveguide sections, filters and other devices are useful in a wide variety of microwave applications. These corrugated devices are difficult to fabricate with accuracy and the higher their frequency of operation, the more difficult it is to obtain the required accuracy. At frequencies exceeding approximately 10 GHz, dimension control of fins, fin spacing and wall thicknesses become difficult and costly. Furthermore, weight of the microwave device becomes a factor of importance in certain applications, such as in satellite communications.
A prior art method for fabricating corrugated horns was electroforming on a mandrel. The mandrel would have the desired taper and slots for fins and after the electroforming of the device onto the mandrel was completed, the mandrel would be removed by chemical etching. This method is in many cases satisfactory for operational frequencies lower than approximately 10 GHz. Above the frequency, accurate fin thickness is difficult to obtain in the mandrel due to the small size of the fins. Also the fin depth is restricted since structurally, the mandrel could only be slotted to a certain depth. Due to these mechanical restrictions, the fin width of depth ratio is limited and this limits the maximum frequency of operation. Since the mandrel was chemically etched away it is not reusable thus adding to the cost of fabrication. Also, the etching process can be lengthy which adds to the cost and lessens the ease of manufacture.
A second method of fabrication used in the prior art is casting. This method has found little application in the higher frequency ranges since required accuracy is extremely difficult or impossible to obtain. Above approximately 10 GHz, it is extremely difficult to obtain the small fin width required. Also, casting molds are relatively expensive.
Another prior art fabrication method is presented in the article entitled: "Characteristics of a Broadband Microwave Corrugated Feed: A Comparison Between Theory and Experiment," by Dragone in The Bell System Technical Journal, Vol. 56, No. 6, July-August 1977, pages 869 to 888. This method is claimed to be a novel fabrication technique usuable at very high frequencies, as high as 100 GHz (page 887). According to this article, a block of sandwiched aluminum and brass disks is assembled. Then an outer surface is machined and a wall of metal is electroplated onto this surface. Then an inner surface is machined. After that machining, the aluminum is removed with a solvent, thus leaving the final product, a corrugated horn. The article analyzes the performance of a feed made in accordance with this fabrication technique at frequencies ranging from 17 GHz to 35 GHz (page 871).
Although it is claimed that a horn operable as high as 100 GHz may be constructed using Dragone's process, (page 887) there are several disadvantages. Because the outside surface is formed and plated first, this plating must be strong enough to support the subsequent machining of the inside surface. Thus a relatively thick plating is necessary, which increases both the weight and size of the corrugated horn. Also, using Dragone's process, horn throat sections, flanges or transitions must be internally machined at the same time as the inner surface. This technique becomes physically difficult or impracticable at frequencies above approximately 20 GHz due to very small apertures and required very close tolerances.
It is a purpose of the invention to provide a simple and reliable method for fabricating corrugated microwave components with a lower manufacturing cost than prior art methods.
It is also a purpose of the invention to provide a method for fabricating corrugated microwave components where more accurate dimension control is possible than prior art methods.
It is also a purpose of the invention to provide a method for fabricating corrugated microwave components where the component can be made lighter than prior art methods permitted.
It is also a purpose of the invention to provide a method for fabricating corrugated microwave components where the fabrication may be completed faster than with prior art methods.
It is also a purpose of the invention to provide a method for fabricating corrugated microwave components which are usable at high frequencies including and exceeding the 100 GHz frequency range.
It is also a purpose of the invention to provide a method for fabricating corrugated microwave components where preconstructed components such as throat sections, flanges or transitions may be added thereby providing an integrated assembly.